Coalescence in Blends of Thermoplastic Polyurethane With ...

Coalescence in Blends of Thermoplastic Polyurethane

With Polyolefins

KATRIN WALLHEINKEϩ and PETRA PÖTSCHKE*

Institut für Polymerforschung Dresden e.V
D-01069 Dresden, Germany

CHRISTOPHER W. MACOSKO

University of Minnesota
Department of Chemical Engineering and Materials Science

Minneapolis, Minnesota 55455

HERBERT STUTZ

BASF AG
Kunststofflaboratorium
D-67056 Ludwigshafen, Germany

The coalescence behavior of immiscible blends was determined by annealing the
melt without shear. The kinetics and the mechanism of coalescence were observed
for blends of thermoplastic polyurethane (TPU) and polyolefins. Two different types
of measurements were used to observe coalescence in quiescent melt at the pro-
cessing temperature. On the one hand, the blend granules were annealed in bulk.
The resulting particle sizes were determined by light microscopy and by SEM on
particles separated from the blend. On the other hand, thin samples were annealed
on a hot stage. Coalescence was observed in situ by light microscopy or static laser
light scattering. It was found that the higher viscosity and elasticity ratios between
polyethylene and TPU lead to a more pronounced coarsening of the morphology of
80/20 blends than in TPU/polypropylene. It has been shown that the process of
reshaping coalescence is one mechanism of coalescence that occurs in quiescent
melt. Another mechanism that was directly observed is a “domino effect” where one
coalescence process causes the next one.

INTRODUCTION critical value, interaction forces between the droplets
cause the film to rupture, and coalescence takes place.
In multiphase polymer systems, the morphology during
melt mixing is a result of the competitive processes This three-step mechanism of coalescence has been
of break-up and coalescence of the dispersed particles confirmed for polymer blends under shear (1, 8, 9). It
(1–4). In this case, coalescence is flow-induced. can be assumed that flow-induced coalescence in these
systems is less pronounced than in the Newtonian
For Newtonian fluids, especially in the case of emul- systems because the viscoelastic properties of the ma-
sions, coalescence in shear flow proceeds in three trix retard film drainage.
steps (5–7). First, the dispersed droplets approach each
other and form a rotating doublet. The drainage of the In quiescent melt, coalescence is a result of the ten-
thin film of fluid between the droplets can require dency of the system to minimize its free energy by re-
considerable time and is the rate determining step for ducing the interfacial area between the components.
coalescence. When the contact time is too short be- The mechanism of coalescence in systems without
cause of high flow rates, the droplets do not coalesce shear is not yet well understood. In the literature, sev-
and separate. As soon as the film thickness reaches a eral explanations are discussed.

*To whom correspondence should be addressed. Ostwald ripening (10, 11) is described to take place
ϩ Present address: Georg Fischer Piping Systems Ltd., CH-8201 Schaffhausen. in blends where the diffusion coefficient of the minor
component in the major phase is sufficiently high.
Brownian motion has been proposed to be one of the
reasons for the approach of particles and the drainage

1022 POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6

Coalescence in Blends of Thermoplastic Polyurethane With Polyolefins

of the matrix film (12). However, it can be calculated tions of the polymers were measured at the process-
from the equation for the movement of particles by ing temperature of TPU (T ϭ 230°C) and corrected
Brownian motion that, because of the high viscosities with the Rabinowitsch-Weissenberg correction.
of most polymers, Brownian motion probably plays a Oscillatory shear measurements were carried out at
minor role in polymer blends. In low molecular sys- 230°C with a plate-plate (25 mm diameter) configura-
tems, sedimentation (Stokes flow) and temperature tion and a gap of 2 mm by use of a Rheometrics
gradients (Marangoni flow) are major causes for parti- Mechanical Spectrometer (RMS 800). The viscosity
cle approach. Søndergaard and Lyngaae-Jørgensen ratio between the dispersed polyolefins and the TPU
(13) showed that these mechanisms do not occur in was calculated at the shear stress ␶ in the matrix as-
polymer blends. sumed to be predominant during extrusion (␶ ϭ 1.6 ϫ
105 Pa, at a shear rate of 200 s–1).
A more probable explanation is based on the as-
sumption that the dispersed phase content is above The surface and interfacial properties of some of the
or close to the percolation threshold (14). This leads to blend components were measured using the pendant
a number of close droplets that can coalesce as a re- drop method. Using this method, the surface tension
sult of weak interactions, most probably the van der of the polyethylene used in the blends was not mea-
Waals forces. This makes it possible to neglect the surable because of its high viscosity and elasticity.
first step of coalescence, the approach of the particles. Therefore, a PE with a lower viscosity (Mirathen A 17
Nevertheless, this theory is still not able to describe MA, Leuna AG) was used. The surface tension of TPU
the experimental results. is not measurable directly in the melt because of its
insufficient thermal stability. Therefore, model sub-
Another approach is assuming that viscous flow (in- stances for the soft segment and hard segment were
terfacially driven coalescence, reshaping agglomera- used (18, 19). Interfacial tensions were determined by
tion) leads to the reduction of the interfacial area (13, forming a drop of the denser polymer in the melt of
15). In the case of irregularly shaped droplets, as they the other material (18).
are usually produced by melt mixing, the droplets will re-
turn to a spherical shape when the system is remelt- Processing
ed. At higher contents of dispersed phase, this leads
to the contact and coalescence of neighboring drops. Prior to blending, the thermoplastic polyurethane
was dried for at least three hours in vacuum at 100°C.
In spite of these theories and experimental results,
coalescence in quiescent melt is still far from being Blending of the components was done by melt mix-
understood. One reason may be that experimental set- ing in a co-rotating, intermeshing twin-screw extruder
ups and blend systems yielding quiescent melt are ZSK 30 (L/D ϭ 32, Werner & Pfleiderer). The screw
scarce. Even without applying shear to the blend, a configuration was adapted for the blend system TPU/
significant amount of flow has been observed in polyolefin. The melt temperature was 230°C, the output
molten blends (16, 17). rate was 10 kg/h with a residence time of about 50 s.

The aim of this paper is to discuss coalescence in If not stated otherwise, the composition of the blend
quiescent melt of thermoplastic polyurethane and was 80 wt% TPU/20 wt% polyolefin.
polyolefin blends. The influence of the type of dis-
persed phase and its content on particle size and coa- Annealing and Analysis of the Morphology
lescence were determined with two experimental set-
ups and different characterization methods. Annealing in Bulk

EXPERIMENTAL Blend granules were annealed in a bath of Wood’s
metal at the processing temperature (230°C). Before-
Materials hand, the granules were dried for 3 hours at 100°C in
a vacuum oven and then wrapped in aluminum foil.
A polyester thermoplastic polyurethane elastomer The annealing time started with the dipping of the
with a shore hardness of 64 D (TPU; Elastollan® C 64 specimen into the metal bath. The specimens were
D, Elastogran GmbH) was used as matrix. The dis- quenched in ice water after annealing in order to
persed phase consisted either of polypropylene (PP; freeze the morphology. The morphological characteri-
Novolen® 1127 MX, BASF AG) or of high density poly- zation of the virgin and the annealed samples was
ethylene (PE, Lupolen® 4261 A, BASF AG). done in two different ways.

The thermoplastic polyurethane was a segmented On the one hand, the specimens were cryo-micro-
block copolymer consisting of hard segments and tomed. The thin sections (3 ␮m) were analyzed with
polyester soft segments. The polypropylene was a ho- phase contrast light microscopy (BH 2, Olympus). The
mopolymer (MFR 230/2.16 ϭ 8 g/(10 min)) recom- microscope was fitted with a CCD camera for digitaliza-
mended for flat-sheet die extrusion. The polyethylene tion of the images for quantitative analysis. The images
had a MFR (190/ 21.6) of about 6 g/(10 min) with the were acquired under comparable conditions (brightness,
main field of application blow molding. contrast) and analyzed quantitatively using an image
analysis software (Optimas 5.1, Optimas Corporation).
The viscosities were determined with a high pres- Manual corrections were minimized to ensure compa-
sure capillary rheometer (HKV), Rheograph 2003 rability of the results. The distribution of the equiva-
(Göttfert). The capillary had a diameter of 1 mm, the
length to diameter ratio was 30. The viscosity func-

POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6 1023

Katrin Wallheinke, Petra Pötschke, Christopher W. Macosko, and Herbert Stutz

lent circle diameter was evaluated. The number average as described above and the intensity of the scattered
mean particle diameter (dn) and the standard deviation light (I ) was measured as a function of the scattering
(sd) as a measure for the broadness of particle size dis- angle (scattering profiles) and the annealing time. The
tribution were determined for at least 1000 particles. data were evaluated using plots of I –1/2 versus q 2 (23).
The scattering vector q is given by:
On the other hand, the TPU was selectively dis-
solved (DMFϩ 5 g/l dibutylamine, room temperature) 4ؒ␲ؒn
from the granules. The resulting dispersion of poly- q ϭ ␭0 ؒ sin1␪>22 (1)
olefinic particles in the TPU solution was separated
using low pressure membrane filtration [for details with n being the refractive index, ␭0 the wavelength of
concerning the method see Wallheinke et al. (20)]. The the laser and ␪ the scattering angle.
particles on the membrane were analyzed using a low
voltage SEM (DSM 982 Gemini, Zeiss) without previ- Under the assumption that no multiple scattering
ous sputtering at an acceleration voltage of 1 kV. takes place, the correlation length ␨ can be determined
from the slope and intercept of the linear extrapola-
Annealing of Thin Sections
tion of the data from these Debye-Bueche plots:
In situ observations of coalescence have been car-
ried out by annealing about 30-␮m-thick sections of slope 1>2
blend granules on a hot stage using either light mi- ␨ϭ a b (2)
croscopy or static laser light scattering. The samples intercept
were prepared by pressing thin sections of the blend
between two cover slides to a thickness of about 20 to The mean particle diameters dscatt were calculated using:
40 ␮m because this size was reported to be optimal
for light scattering experiments on blends (21, 22). 3ؒ␨ (3)
The samples were annealed at 230°C on a hot stage. dscatt ϭ 2 ؒ 11 Ϫ ␾ 2
The temperature gradient of the hot stage (Mettler)
was found to be less than 1°C. ␾ is the content of the dispersed phase.
In addition, annealing experiments on thin sections
One-dimensional laser light scattering was done
using a scattering setup similar to that described by were carried out using a videocamera on the micro-
Okamoto and Inoue (21). It consisted of a He-Ne laser scope (transmitted light, bright field) and recorded on
(wavelength 632.8 nm), parallel polarizers, the hot a videotape for later digitalization of pictures.
stage, and an array of 38 photodiodes (Hamamatsu,
S 4112-series). The signals from the diodes were ampli- RESULTS AND DISCUSSION
fied prior to digitalization. The samples were prepared
Properties of the Blend Components

The rheological behavior of the blend components
measured at 230°C is shown in Figs. 1 and 2. Polyeth-

Fig. 1. Viscosity functions of the blend components measured with a high pressure capillary rheometer and a mechanical spectrom-
eter at 230°C.

1024 POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6

Coalescence in Blends of Thermoplastic Polyurethane With Polyolefins
Fig. 2. Storage (GЈ) and loss modulus (GЉ) of polyethylene and polypropylene.

Fig. 3. Dependence of the surface tensions of the polyolefins and of model substances for the TPU segments on the melt temperature
measured by pendant drop analysis.

ylene has the highest viscosity of the blend compo- the storage (GЈ) and loss (GЉ) moduli of the polyolefins.
nents (Fig. 1), followed by TPU and PP. Therefore, the In the PE, GЈ ist larger than GЉ. In contrast, the vis-
viscosity ratio of PE to TPU is larger than 1 over the cous behavior dominates in the PP.
whole range of the shear rate whereas that of PP to
TPU is lower than 1. At the predominant shear stress In Fig. 3, the dependence of the surface tension on
in the matrix during dispersion, the viscosity ratio of temperature as measured by pendant drop analysis is
PP to TPU is 0.03, of PE to TPU 3.54. Figure 2 shows shown. The surface tensions of the two polyolefins are
identical, the surface tension of the soft segments is

POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6 1025

Katrin Wallheinke, Petra Pötschke, Christopher W. Macosko, and Herbert Stutz

Fig. 4. Number average particle diameter dn and broadness of the particle size distribution as a function of the PP and PE concentra-
tion in the blend (determined by light microscopy).

significantly lower than that of the hard segments. It (represented by the standard deviation sd) are shown
is known from literature that the TPU soft segments as a function of the PP and PE concentration. In both
are segregated at the interface in blends with unpolar systems, a marked increase of the particle diameter
polymers (24). Therefore, the interfacial tension is de- sets in at dispersed phase contents of about 1 wt%.
termined by the surface properties of the soft seg- Another interesting point to observe is that the parti-
ment. The interfacial tension of the TPU soft segment cle size at very low dispersed phase contents is differ-
with PE was measured directly (19, 25) but measure- ent for the two systems. In the blends of TPU with 0.1
ments failed for the TPU soft segment with PP. There- wt% of PE it is 0.77 ␮m, with PP 0.55 ␮m.
fore, the harmonic-mean equation (26) was used to
determine the interfacial tension between these com- The limiting particle size without coalescence for in-
ponents (25), assuming a polarity of 0.02 for the PP finite dilution in Newtonian systems under shear is
(26). The interfacial tension between the soft segment given by the Taylor equilibrium drop size dT (31):
and PE is 9.4 mN/m, with PP it is about 8.2 mN/m.
Owing to these results, only slight interactions in the dT ϭ 4 ؒ ␴12 ؒ 1␭ ϩ 1 2 (4)
interface of the two components in the blends can be ␩m ؒ
expected. ␥# ؒ c a 19 ؒ ␭b ϩ 4d
4
The lack of interactions between TPU and the poly-
olefins was confirmed by SEM of cryofractures, dy- The limiting particle size depends on the interfacial
namic mechanical analysis, differential scanning
calorimetry, and the complete separation of the blend tension ␴12, the viscosity ratio ␭ at the predominant
into its components from solution (27). shear stress ␶ϭ ␥• ␩m in the mixer, and the matrix vis-

Influence of the Dispersed Phase Content cosity ␩m. Under these conditions, dT is 0.052 ␮m for
on the Morphology of Granules
both PE and PP. In the range of shear rates applied
From the literature it is known that the particle size
and the broadness of the particle size distribution during extrusion (5 – 5000 s–1), the Taylor equilibrium
increase significantly with a rising content of the
dispersed phase (28, 29). This is due to the onset of drop size varies from 0.013 to 0.46 ␮m, and is always
coalescence at dispersed phase contents of more than
1 wt% (1, 30). very similar for the two polyolefins. This shows that

In Fig. 4, the number average particle diameter dn the viscoelastic properties of the PP and the PE cause
and the broadness of the particle size distribution
the differences between the measured particle sizes at

0.1 wt% and dT . The larger particle size in the PE
compared to the PP is probably due to the viscosity

ratio. At shear rates smaller than approximately 100

s–1, the viscosity ratio of PE with TPU becomes larger

than 4. According to Grace (32), breakup of drops in

simple shear is impossible at these viscosity ratios.

This means that in the regions of lower shear rates in

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Coalescence in Blends of Thermoplastic Polyurethane With Polyolefins

(a) In addition, Fig. 4 shows that the increase of the
particle size with composition compared to the lowest
(b) particle size is much more pronounced in the blend
with polyethylene as dispersed phase. This is proba-
(c) bly due to the big differences in the viscoelastic prop-
erties between PP and PE.
Fig. 5. SEM micrographs of PE particles separated from the
blends with PE concentrations of (a) 0.1 wt%, (b) 3.33 wt%, Fig. 5 shows SEM micrographs of PE particles sepa-
and (c) 10 wt% using low pressure membrane filtration. rated from blends having different PE concentrations:
the extruder, PE drops are deformed but do not break, 0.1, 3.33, and 10 wt%. The oriented structure visible
whereas the PP drops are subject to breakup over the in the background is the PP-membrane on which the
whole range of the shear rate. particles were deposited. The size differences between
the particles from the blends are obvious but another
striking point is that at a PE concentration of 10 wt%
(Fig. 4c ), particles in the state of coalescing have been
frozen in before reshaping could take place.

Annealing in Quiescent Melt

Annealing of Granules

Annealing of 80 wt% TPU/20 wt% polyolefin gran-
ules in the melt at the processing temperature of
230°C leads to a marked coarsening of morphology in
both blends. The quantitative analysis of light micro-
graphs is shown in Fig. 6. In the blend with dispersed
PP, the number average particle diameter doubled in
30 minutes. The particle size in the TPU/ PE blend in-
creased fourfold after 30 minutes of annealing. These
results confirm the marked difference in the amount of
coalescence during blending that has been discussed
in the previous paragraph. Besides the changes in the
mean particle size, the increase of the broadness of
the size distribution indicates that small particles still
exist after annealing.

SEM micrographs of PP particles separated from the
virgin granules of TPU/PP 80/20 (Fig. 7a) and from
granules annealed for 30 minutes (Fig. 7b) confirm
this result (see small particles in the upper right cor-
ner of Fig. 7b). From these micrographs, another in-
teresting observation can be made. In the virgin gran-
ule, the fibers of PP with Rayleigh instabilities were
frozen during the process of breakup. Upon reheating
during annealing, these fibers either break up to
spherical particles or reshape to a sphere without
breaking. In both cases, the reshaping leads to an in-
crease in the diameter, which implies the possibility of
a collision with neighboring particles leading to re-
shaping coalescence (Fig. 8). As expected, all elongat-
ed particles have disappeared after annealing for 30
minutes (Fig. 7b).

Measurements of the relative viscosity of TPU in
DMF (concentration 0.005 g/cm3) show that the mole-
cular weight of the TPU was not affected by annealing.
This indicates that the rate of coalescence is not influ-
enced by a change in the viscosity of the TPU.

Annealing of Thin Sections on a Hot Stage

TPU/PP 80/20 blends: Figure 9a shows the inten-
sity of the light scattered by a TPU/PP 80/20 thin sec-
tion as a function of the scattering angle during an-

POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6 1027

Katrin Wallheinke, Petra Pötschke, Christopher W. Macosko, and Herbert Stutz

Fig. 6. Number average mean particle diameter dn and broadness of the particle size distribution of TPU/PP and TPU/PE 80/20
blends in dependence on the annealing time of granules in a metal bath.

(a) (b)

Fig. 7. SEM micrographs of PP particles separated from TPU/PP 80/20 granules before (a) and after (b) 30 minutes of annealing at
230°C in a metal bath.

nealing at 230°C. To eliminate errors induced by ma- (Fig. 9c). The increase in the particle size is similar to
trix scattering, the intensity values used for the deter- that obtained by annealing in bulk, the mean particle
mination of the particle size were corrected by the in- diameter nearly doubled after 20 minutes of anneal-
tensity values of the pure TPU at the same annealing ing. The biggest difference was observed in the ab-
times. Typical shapes of the scattering profiles for solute values that have been measured (see also Fig.
polymer blends were observed. The slope of the curve 13). The particle size in the light scattering sample be-
changes with annealing time. The intensity of small fore annealing was extrapolated to be about 0.4 ␮m,
particle scattering (at high angles) decreases and the compared with the number average diameter of 1.6
intensity of the big particle scattering increases, ␮m measured by light microscopy on thin sections of
which is an indication for coalescence. the virgin granule. The reasons for this are assumed
to be the difficulties in calibrating the light scattering
From the linear regressions of the Debye-Bueche setup, multiple scattering, and the differences in the
plots (Fig. 9b) the mean particle size was determined

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Coalescence in Blends of Thermoplastic Polyurethane With Polyolefins

Fig. 8. Scheme of the process of reshaping coalescence.

particle size range for which the two methods are ap- in the sample owing to the high viscosity of the TPU.
plicable. Light microscopy neglects particles smaller This result confirms that it is reasonable to discuss
than 0.3 ␮m and overestimates the true particle size. quiescent melt in samples of this blend. On the other
In contrast, laser light scattering gives reasonable values hand, a mechanism has been observed that is similar
up to a maximum particle size of about 3 ␮m. Therefore, to that of reshaping coalescence. Figure 11 shows
light scattering underestimates the particle size. higher magnifications of the same annealing experi-
ment shown in Figure 10. Two particles that are in
To obtain more information about the changes in close contact are coalescing. The coalescence process
the morphology during annealing of the thin sections, itself takes about 3 seconds. The newly formed parti-
the experiments were repeated using a light micro- cle gets close to a neighboring particle because of the
scope. Figure 10 shows micrographs acquired from increase of its diameter after reshaping to a sphere.
videotapes that were recorded during the experi- These particles are close to each other without any
ments. It can be seen that the morphology coarsens visible changes, but after some minutes, they coalesce
significantly. In addition, the causes for coalescence spontaneously. A schematic sketch of this mechanism
in this sample were observed directly. On the one of coalescence is shown in Fig. 12. As one coalescence
hand, it can be seen that there is no significant flow

(a)

Fig. 9. (a) Scattering profiles taken during the annealing of TPU/PP 80/20 at 230°C, (b) resulting Debye-Bueche plots, (c) particle
sizes in dependence on the annealing time.

POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6 1029

Katrin Wallheinke, Petra Pötschke, Christopher W. Macosko, and Herbert Stutz
(b)

1030 (c)

Fig. 9. Continued.

POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6

Coalescence in Blends of Thermoplastic Polyurethane With Polyolefins
Fig. 10. Light micrographs taken during the annealing of TPU/PP 80/20 at 230°C on a hotstage.

Fig. 11. Mechanism of coalescence: “domino effect” observed during annealing of TPU/PP 80/20 at 230°C on a hotstage.

POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6 1031

Katrin Wallheinke, Petra Pötschke, Christopher W. Macosko, and Herbert Stutz
Fig. 12. Scheme of coalescence by the “domino effect.”

Fig. 13. Comparison of light scattering and light microscopy for the determination of the particle size of a TPU/PP 80/20 blend.

event causes the next one, the process is similar to a graph shows a cryofracture of a TPU/PP 80/20 thin
chain reaction and continues until the interparticle section between two cover glasses that had been an-
distances become too long for coalescence. nealed at 230°C for 1 hour. Notable is the particle
with the diameter that is larger than the sample thick-
In order to compare the kinetics of coalescence in ness. It has flattened out against the cover glasses,
bulk (annealing of granules) and in a thin section an which falsifies the results of a particle size analysis.
evaluation of the light micrographs (Fig. 10) was at-
tempted. Because of the thickness of the sample TPU/PE with low dispersed phase content: In the
(about 30 ␮m), this quantitative analysis of the light previous paragraph, it was shown that the particle
micrographs was feasible only for the blends annealed size in TPU/PP blends was too large to give reasonable
for longer times and had to be done manually. results with light scattering. Therefore, some experi-
ments were done with blends with a small content of
In Fig. 13, the results of this analysis are compared dispersed phase where light microscopy has shown
with the results of the annealing in bulk and of the that the particle sizes are much smaller (Fig. 4). The
light scattering on a thin section. The higher particle results of the annealing of TPU/PE 99.9/0.1 and
size in thin sections after 30 minutes of annealing is 96.67/3.33 blends are shown in Fig. 15. The particle
caused by the difficulties in the quantitative analysis size in the blend with 0.1 wt% of PE is much smaller
of light micrographs in which smaller particles were than at 3.33 wt%, which confirms the tendency shown
hardly measurable. Another explanation for the larger by light microscopy (Fig. 4). In addition, it can be seen
particle size determined for samples annealed in a that coalescence cannot occur at the lowest content of
thin section can be seen in Fig. 14. The SEM micro-

1032 POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6

Coalescence in Blends of Thermoplastic Polyurethane With Polyolefins
Fig. 14. SEM micrograph of a cryofracture of a TPU/PP 80/20 blend annealed on the hot stage at 230°C for 1 hour.

Fig. 15. Influence of the dispersed phase content on coalescence in TPU/PE blends determined by light scattering.

dispersed phase, whereas there is already some coa- coalescence begins to occur at dispersed phase con-
lescence at 3.33 wt% of PE. tents of about 1 wt%. The particle sizes at very low
content of dispersed phase and the increase of the
CONCLUSIONS particle size with rising content of polyolefin show a
significant difference for polypropylene and polyethyl-
Coalescence was studied in the immiscible system ene. As the interfacial tensions of the two polyolefins
of thermoplastic polyurethane and polyolefins. As ex- with TPU were shown to be very similar using pen-
pected, the morphology of blend granules depends on dant drop analysis, these differences are mainly
the concentration of the dispersed phase. During blend- caused by the difference in their rheological behavior.
ing of TPU with polyolefins in a twin-screw extruder,

POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6 1033

Katrin Wallheinke, Petra Pötschke, Christopher W. Macosko, and Herbert Stutz

The polyethylene is much more viscous and elastic ␭0 Wavelength of the laser
than the polypropylene, leading to higher viscosity ␴12 Interfacial tension between materials 1 and 2
and elasticity ratios with TPU. The particle sizes in the
TPU/PE blend are larger and coalescence is much ␶ Shear stress
more pronounced than in TPU/PP.
␾ Volume fraction of dispersed phase
Annealing experiments showed that the blend mor-
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1034 POLYMER ENGINEERING AND SCIENCE, JUNE 1999, Vol. 39, No. 6